86 research outputs found
NuSTAR and Swift observations of the ultraluminous X-ray source IC 342 X-1 in 2016: witnessing spectral evolution
We report on an X-ray observing campaign of the ultraluminous X-ray source IC
342 X-1 with NuSTAR and Swift in 2016 October, in which we captured the very
moment when the source showed spectral variation. The Swift/XRT spectrum
obtained in October 9--11 has a power-law shape and is consistent with those
observed in the coordinated XMM-Newton and NuSTAR observations in 2012. In
October 16--17, when the 3--10 keV flux became 4 times higher, we
performed simultaneous NuSTAR and Swift observations. In this epoch, the source
showed a more round-shaped spectrum like that seen with ASCA 23 years ago.
Thanks to the wide energy coverage and high sensitivity of NuSTAR, we obtained
hard X-ray data covering up to 30 keV for the first time during the high
luminosity state of IC 342 X-1. The observed spectrum has a broader profile
than the multi-color disk blackbody model. The X-ray flux decreased again in
the last several hours of the NuSTAR observation, when the spectral shape
approached those seen in 2012 and 2016 October 9--11. The spectra obtained in
our observations and in 2012 can be commonly described with disk emission and
its Comptonization in cool ( keV), optically-thick () plasma. The spectral turnover seen at around 5--10 keV shifts to
higher energies as the X-ray luminosity decreases. This behavior is consistent
with that predicted from recent numerical simulations of super-Eddington
accretion flows with Compton-thick outflows. We suggest that the spectral
evolution observed in IC 342 X-1 can be explained by a smooth change in mass
accretion rate.Comment: 10 pages, 6 figures, accepted for publication in Ap
Discovery of Dying Active Galactic Nucleus in Arp 187: Experience of Drastic Luminosity Decline within years
Arp 187 is one of the fading active galactic nuclei (AGN), whose AGN activity
is currently decreasing in luminosity. We investigate the observational
signatures of AGN in Arp 187, which trace various physical scales from less
than 0.1 pc to the nearly 10 kpc, to estimate the longterm luminosity change
over years. The VLA 5 GHz, 8 GHz, and the ALMA 133 GHz images reveal
bimodal jet lobes with 5 kpc size and the absence of the central
radio-core. The 6dF optical spectrum shows that Arp 187 hosts narrow line
region with the estimated size of 1 kpc, and the line strengths give the
AGN luminosity of erg s. On the other
hand, the current AGN activity estimated from the AGN torus emission gives the
upper bound of erg s. The absence of
the radio-core gives the more strict upper bound of the current AGN luminosity
of erg s, suggesting that the central
engine is already quenched. These multi-wavelength signatures indicate that Arp
187 hosts a "dying" AGN: the central engine is already dead, but the large
scale AGN indicators are still observable as the remnant of the past AGN
activity. The central engine has experienced the drastic luminosity decline by
a factor of fainter within years, which is roughly
consistent with the viscous timescale of the inner part of the accretion disk
within 500 years.Comment: 15 pages, 8 figures, accepted for publication in Ap
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